The invention relates to a method of non-dispersive analysis of gas mixtures for the determination of the concentration of a gas component present therein, the method comprising the following steps; a radiation source is arranged which emits, through the gas mixture being analyzed, radiation within a wavelength range within which the absorption spectrum band used in the determination of the concentration of the said gas component is located; an optical transmission band filter the transmission band of which coincides with the said spectrum band is provided in the path of the radiation traversing the gas mixture being analyzed; a detector is used for detecting the radiation which has traversed the gas mixture being analyzed and the optical transmission band filter, this first intensity generating in the detector a first signal; at least one different intensity is produced in the detector by means of an additional gas or an additional gas mixture interposed between the radiation source and the detector, by using radiation which has also traversed the gas mixture being analyzed and the transmission band filter, this second intensity generating in the detector a second signal; the result of the concentration measurement is formed computationally by using the different measurement signals. The invention also relates to the use of this method and to a sensor means used in its application.
In measuring the concentration of a gas by the infrared technique, the method most commonly used is a non-dispersive method, i.e. the absorption signal is measured through an optical transmission band filter having a transmission bandwidth typically in the same order of magnitude as the width of the spectrum band used for measuring the concentration of the gas component being measured. The measured radiation signal is in this case the integrated value of the transmissions at the different wavelengths of the transmission band. Such devices are described in publications U.S. Pat. No. 3,745,349 and HEWLETT-PACKARD JOURNAL, September 1981, pp. 3-5: R. J. Solomon--"A Reliable, Accurate CO.sub.2 Analyzer for Medical Use." In the former publication, two infrared radiation sources emitting through the sample gas are used, but the radiation from one radiation source is rendered, for example by means of an optical gas filter, narrower than the wavelength range used in the measurement. The purpose is to provide a reference source issuing a radiation intensity which is not affected by the gas mixture being analyzed in the detector. The objective is to compensate for the effect of dirt and other substances accumulating in the measuring chamber and having absorption within the same range as the gas being measured, and to avoid the problems of matching different detectors. Modulated infrared sources are used, in which case an actual signal and a reference signal are obtained alternately in one and the same detector, signals which have thus traversed the same path and detect impurities in the same manner. In the former publication these problems have been solved by using a structure having no moving parts. In the latter publication, an attempt has been made to overcome the same problems but by using a rotating disk which contains different filters for obtaining a reference value.
The absorption spectrum of a gas in molecular form normally consists of absorption spectrum bands produced by molecular vibrations and, within them, a fine structure, i.e. absorption lines, due to rotational transitions. When measured with sufficient discrimination, the absorption spectrum band of a gas is thus made up of a large number of very narrow absorption lines. For example, carbon dioxide has a molecular vibration absorption spectrum band having a mean wavelength of 4260 nm. A more detailed analysis shows that the region is made up of more than 80 narrow absorption lines caused by rotation. The half-intensity linewidth and intensity of these lines are dependent on many factors, such as temperature, self-absorption due to the long measuring path, and collisions by other molecules present in the gas mixture. In measurement signal compensation on the first two can in general easily be taken into account by measuring the temperature and the linearization effects due to the measurement geometry on the gas concerned. On the other hand, the change, sometimes significant, due to collisions by other gas components must be taken into account specifically in order to minimize concentration errors. Changes in the half-intensity linewidth of carbon dioxide in a nitrogen mixture and an oxygen mixture are described in the publication APPLIED OPTICS, Vol. 25, No. 14/1986 pp. 2434-2439. Cousin, Le Doucen, Houdeau, Boulet, Henry--"Air broadened linewidths, intensities, and spectral line shapes for CO.sub.2 at 4.3 .mu.m in the region of the AMTS instrument." The half-intensity linewidth of the carbon dioxide line (ordinal number 67) of a gas mixture at normal pressure is, in an oxygen mixture, 0.055 cm.sup.-1 (0.10 nm) and, in a nitrogen mixture, 0.060 cm.sup.-1 (0.11 nm) for a concentration of 5% CO.sub.2. The portion of self-broadening by carbon dioxide in these figures is only approx. 0.003 cm.sup.-1.
Polar gases such as nitrous oxide have a much greater effect on the half-intensity linewidth than nitrogen and oxygen, discussed in the above-mentioned publication. For this reason, for example, the measurement result of the amount of carbon dioxide in a patient's breathing gas is corrected, for example, by measuring the laughing gas concentration, as in publication U.S. Pat. No. 4,423,739 and by using this result computationally to correct the carbon dioxide concentration. This method is not very reliable, for according to it, it is necessary to know, for example, all the gas components affecting the broadening of the absorption lines, their concentrations must be measured, and ample experimental material must be obtained for the correction calculations. The procedure described in the publication yields a completely erroneous result if there is some factor unknown in advance relating to the gas mixture being analyzed. It is known that the effect of oxygen on the measurement result of nitrogen can be corrected in a manner similar to that mentioned in the said patent, although the error is smaller. By the procedures described in publication U.S. Pat. No. 3,745,349 and in the said article in publication HEWLETT-PACKARD JOURNAL the problem described above cannot be solved, and it has not been discussed in them.
The concentration of a gas is proportional to the number of gas molecules at the measurement volume and pressure. The number of molecules participating in infrared absorption is retained more or less unchanged in a collision process if the conditions do not otherwise change. Only the distribution of energy is slightly changed, causing a broadening of the absorption line. The absorbance value integrated across the absorption line is thus retained practically unchanged. However, by the infrared technique it is not possible to measure absorbance directly; instead, transmission is measured. According to the Lambert-Beer law, EQU T=10.sup.-8
where T is transmission and a is absorbance, applies to one wavelength. Only linearization yields an absorbance value proportional to the concentration: EQU a=-log T.
Especially when the measuring is carried out non-dispersively within a certain bandwidth, the total transmission signal Tm will be an integral across the spectrum range of the filter .lambda.1-.lambda.2: ##EQU1## where F(.lambda.) is the wavelength-dependent transmission function of the filter and T(.lambda.) is the wavelength-dependent transmission function of the gas sample. This signal is linearized experimentally, since the Lambert-Beer law no longer applies. The end result is usually different from the total absorbance A, which is in practice independent of collision broadening and is an integral across absorbances a(.lambda.): ##EQU2## In fact, the size of the error in Tm depends on the bandwidth of the filter. Measured by using a very narrow transmission band watch is in the same order as the total width (e.g. in the order of 0.1-1.5 nm) of an individual absorption line in the absorption band, the need for correcting the collision broadening is very small or nil. On the other hand, if the transmission band of the filter is even narrower (e.g. less than approx. 0.05-0.5 nm) than the full width of a single absorption line being measured, collision broadening causes a reduction of the signal corresponding to the concentration, since the absorption peak becomes lower. Such a case is reported in publication WO-94/24528, in which one absorption peak of oxygen is measured by using a very narrow-band laser diode. In this case, however, the Lambert-Beer law applies with respect to the laser wavelength, and after linearization the absorbance can be integrated as a function of the wavelength, so that the collision broadening can be compensated for.
If the filter band extends over a plurality of absorption peaks, as is usual in the measuring of carbon dioxide, there is need for correction, since the concentration reading increases as the collision broadening increases. Thus the problem caused by the broadening of the absorption lines is not solved by the arrangement described in publication EP-405 841, wherein two detections are used of which one is approximately within one half of the absorption spectrum band and at a concentration deviating (either high and/or low) from the measuring range of the gas being measured, and the other one is within the other half of the absorption spectrum band and at the analyzed concentration of the gas being measured, the result concentration being calculated on the basis of these measurements. Thus, for example, when the question is of certain isotopes of carbon dioxide, the measurements are carried out through transmission band filters of which one has a band of 4.0-4.4 .mu.m and the other one a band of 4.2-4.6 .mu.m. It is known that the extreme limits of the spectrum band concerned are 3.5 .mu.m and 4.7 .mu.m, and thus it must be noted that each range contains a large but indefinite number of absorption lines, in which case the situation is not necessarily always under control. The concentration error is also affected by the length of the measuring channel, i.e. that dimension of the measuring chamber in which radiation traverses it. In the case of a short channel the need for compensation is smaller, whereas in the case of a longer channel, in which the absorption of the gas being measured has reduced the transmission to a substantially greater degree, the need for compensation can be considerable. In the measuring of carbon dioxide in a patient's breathing air, in which for example a considerable portion of the gas mixture may be nitrous oxide, i.e. laughing gas, a concentration value measured and calculated in a conventional manner may be up to 15% too high, owing to the error explained above, due to the transmission integral.